The present invention relates to beamforming in ultrasound imaging systems and, more particularly, to dynamically focused digital beamformers which use delta-sigma modulators.
A conventional ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of directivity of the associated transmit and receive beam pair.
In a typical ultrasound imaging system, the output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Conventional ultrasound beamformers use dynamic focusing during reception of echoes. With this method, the beamformation process is optimized for each depth to achieve as good a beamshape (i.e., narrow beamwidth with low sidelobes) as possible. In most systems, a single fixed focus is used during transmit beamformation to try to maintain a good combined beamshape. In areas away from the transmit focus, the beamwidth of the resultant beam enlarges and the sidelobes increase. In the manufacture of an ultrasound system, the beamformer control is installed in either algorithmic or tabulated form.
In many conventional ultrasound imaging systems, time delay resolution in the beamforming circuitry of the receiver requires a large amount of hardware and consumes a large amount of power. The most recent designs sample each transducer element output signal using very accurate analog-to-digital converters (ADCs) which produce multi-bit digital numbers. These multi-bit numbers are delayed by separate circuitry, including first-in/first-out (FIFO) registers, decimators, and interpolators, before being summed with the separately delayed, multi-bit signals from each of the other transducer elements. This is a considerable amount of hardware for a conventional 128-element transducer array, and is an enormous amount of hardware when a two-dimensional transducer array having 512 or more elements is considered.
A delta-sigma modulator or converter is a circuit which converts an analog signal into a digital signal stream. Delta-sigma (xcex94-xcexa3) analog-to-digital converters have been proposed to radically reduce the size, cost and power consumption of digital ultrasound beamformers. Noujaim et al. U.S. Pat. No. 5,203,335, issued Apr. 20, 1993 and assigned to the instant assignee, sets forth advantages of the xcex94-xcexa3 converter for digital beamforming. The xcex94-xcexa3 converter produces a data stream with a small number of bitsxe2x80x94in its purest form, a single bit at a data rate much higher than the Nyquist sampling frequency of the input signal. The converter output signal can be converted into a more familiar multiple-bit data stream, such as that produced by the traditional ADC, by filtering and decimating in time. To a reasonable approximation, the capacity of a digital data stream is nR, where n is the number of bits in the data and R is the data sample rate. Thus a 1-bit xcex94-xcexa3 converter running at 320 MHz is roughly the equivalent of an 8-bit, 40-MHz ADC.
The relatively high data rate of the xcex94-xcexa3 converter is attractive for digital time-delay beamforming. In its simplest form, a digital beamformer delays the digital data stream of each channel according to a receive focusing schedule, then sums over all the delayed data streams to produce a focused xe2x80x9cbeamsummedxe2x80x9d signal. In this simple form, the resolution of the time delays which the beamformer can generate is the data sampling time interval. The 8-bit, 40-MHz ADC used in the example above is representative of what is currently available for ultrasound imagers. A sampling rate of 1/(40xc3x97106MHz)= 25 nsec is inadequate for all but the lowest imaging frequencies, so that various interpolation schemes are required. These increase the cost and complexity of the integrated circuit which implements the digital delay.
The sampling rate of the equivalent one-bit xcex94-xcexa3 converter is eight times higher, which is adequate for imaging at center frequencies up to about 10 MHz. A beamformer could simply delay, without interpolation, the xcex94-xcexa3 data streams. The beamsummed data would be filtered and decimated to reconstruct a multiple-bit signal which would be passed to the display processor of the imager.
The simplicity of the xcex94-xcexa3 beamformer architecture enables a reduced size with respect to the equivalent beamformer based upon ADCs. This means that the beamformer hardware could be moved from the ultrasound console into the probe itself. This has a number of important advantages. Currently a shielded signal line is required to connect each beamformer channel to the console. The bulk of these cables is a serious obstacle to increasing the channel count in ultrasound imagers. With a xcex94-xcexa3 beamformer in the probe, the channel signals are not brought to the console, so that the cable count is dramatically reduced. This reduces the transducer cable complexity (size and weight). Noise pickup in the cable is a serious concern currently, since the cable carries low-level analog signals. In a xcex94-xcexa3 beamformer, only the beamsummed digital signal need be transmitted to the console.
However, xcex94-xcexa3 beamformers are beset by a difficulty in practice. Delaying a digital signal by one sample, for example, requires insertion of some sample into the data stream. With a multiple-bit data stream, a sample could simply be repeated when the data stream is to be delayed. This introduces some waveform distortion on the scale of the change in the waveform over the sampling time interval, which is a relatively small error when the sampling time interval is sufficiently small. (In practice, traditional ultrasound machines implement delays using a combination of repeated sample and interpolation to relax the requirement on sampling time interval.)
The artifact produced by repeating a single-bit xcex94-xcexa3 sample is more severe, however. The reason is that the single-bit xcex94-xcexa3 data stream must at some point be low-pass-filtered to reconstruct a multiple-bit waveform. The relative error introduced by repeating a xcex94-xcexa3 sample is of order one divided by the filter length. The filter length will be of order of the oversampling ratio, which is currently limited by circuit speed to about 32 for a 10-MHz transducer frequency. This inserted sample, therefore, produces a distortion in the output signal of order 20log({fraction (1/32)}) or about xe2x88x9230 dB. This is unacceptable for all but the least expensive of beamformers.
Freeman et al., in a paper entitled xe2x80x9cAn Ultrasound Beamformer Using Oversampling,xe2x80x9d 1997 IEEE Ultrasonics Symposium Proceedings, proposed three methods to correct the dynamic focus artifact. In one method, they proposed using a two-bit xcex94-xcexa3 converter, which produces a zero level in addition to the +1 and xe2x88x921 levels of a one-bit xcex94-xcexa3 converter. This zero level is used when a sample must be inserted, which reduces the size of the artifact that survives the reconstruction filter. In a second method, they interpret the four levels of a two-bit converter as xe2x88x921, {fraction (xe2x88x921/2)}, +xc2xd, and +1. They divide in half the sample to be repeated and spread it over two inserted samples. In the third method, they manipulate the gain in the feedback loop of the xcex94-xcexa3 converter to compensate for repeating a sample. There is a need for methods of correcting the dynamic focus artifact produced in xcex94-xcexa3 beamforming without requiring the complexity of a two-bit converter or manipulating the gain in the feedback loop.
In a preferred embodiment of the invention, artifacts resulting from time-delay focusing are minimized while the single-bit xcex94-xcexa3 converter for each channel is retained. This simplifies the architecture of the beamformer hardware, reducing its size and complexity. Preferred embodiments based on three different approaches are disclosed. Because there are advantages and disadvantages to each approach, the proper choice for a given application depends upon a cost-performance analysis.
In accordance with a first preferred embodiment of the invention, the dynamic focus artifact in a xcex94-xcexa3 beamformer is corrected using a method wherein a pair of samples, +1 and xe2x88x921, are always inserted when a delay is required. This approach is similar to Freeman""s approach, discussed above, of inserting a zero when a sample delay is required, but requires only a single-bit converter. Although the effective time-delay quantization interval is doubled relative to that in the Freeman teaching, for many applications this will not matter. This method reduces the focusing artifact on each beamforming channel.
In accordance with two other preferred embodiments of the invention, the delay artifact is not reduced on each beamforming channel, but rather is reduced in the beamsummed, reconstructed signal. In many applications, the range-dependent part of the focusing time delay is, to a good approximation, a function of the distance of an element from the aperture center. In this situation, focusing time delays always occur simultaneously on pairs of channels connected to pairs of transducer elements lying equidistant on either side of the aperture center. Thus, in accordance with a second preferred embodiment of the invention, a +1 is inserted on one channel and a xe2x88x921 is inserted on the other channel of a pair of symmetric channels. The net effect on the beamsummed signal is zero, exactly as if a zero had been inserted on each of the two channels.
In other situations, it may not be possible or convenient to arrange the channels in such pairwise fashion. Thus, in accordance with a third preferred embodiment of the invention, a +1 sample is inserted when a focus time delay is required. This method either pre-calculates or counts the number of channels which are delayed at each time sample and then subtracts this number from the beamsum. The net effect on the beamsum is the same as inserting a zero sample on every channel which is delayed. Pre-calculating the delay count is a simple matter, but the added control and memory requirements must be weighed against the additional circuitry required to count the number of focus time delays on each sample. If in a particular implementation it is determined that counting the focus time delays is the better approach, then the focus time delay count can be either exact or approximate.